Accurate characterization of wavefronts produced by an eye is desirable in the field of ophthalmology to facilitate correction of an eye's image-forming system through surgery and/or corrective lens fabrication.
Although various types of aberration measurement apparatus (hereinafter, “aberrometers”) are known, Hartmann-Shack type aberrometers are widely used in commercial ophthalmic applications. FIG. 1 is a simplified schematic illustration of an example of a Hartmann Shack aberrometer 100.
In use, a beam of light from a light source 110 in the aberrometer is directed toward the cornea C of an eye E and onto the retina R by beam splitter 120. The light reflects from the retina and is projected through the cornea, and forms an aberrated wavefront. The aberrated wavefront reenters the aberrometer, and is incident on an array of lenslets 130. The light forms an array spots d11-d1n on sensor 140. The locations of the spots relative to the locations that spots would have occupied in the absence of wavefront aberrations provides data that is used to characterize the wavefront and thus detect aberrations. FIG. 2 is a graphical illustration of example intensity values on a representative area of sensor 140 (including a plurality of spots di,j).
A seminal reference in the field of ophthalmic wavefront detection is Liang et al., Objective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor, Journal of the Optical Society of America, Vol. 11, No. 7, pp. 1-9 (July 1994), the disclosure of which is hereby incorporated by reference in its entirety. Improvements to the technique of Liang et al., id., are taught in Liang and Williams, Aberrations and retinal image quality of the normal human eye, Journal of the Optical Society of America, Vol. 4, No. 11, pp. 2873-2883 (November 1997), and in Williams et al. U.S. Pat. No. 5,777,719, the disclosures of which are hereby incorporated by reference in their entireties.
The ability to accurately measure aberrations and use the measurement information in corrective applications depends on the ability to precisely determine the location of the centers of the spots associated with each lenslet in an array. An inability to accurately detect the centers of all image spots frustrates the characterization of the wave aberrations and subsequent procedures that rely upon those characterizations.
Typically, center coordinates cx, cy of an image spot are calculated by centroid calculation (i.e., summation of weighted values of the incident light intensity I(xi, yi) at points (xi, yi) on sensor 140). Many known factors operate to frustrate accurate centroid determination. For example, scattered light (i.e., noise) from the aberrometer componentry or from the eye itself can form ghost images and/or create background light on the detector that interferes with actual image spot detection and subsequent centroid determination. Image processing techniques that employ high band-pass filtering or certain linear filters may provide a reduced noise component; however, such filtering may also create significant edge distortion and/or may alter the size and shape of a feature of the image (e.g., due to aliasing or ringing).